While they fall under the purview of state pharmacy boards, critics contend those state agencies are often too understaffed and underfunded to adequately monitor such facilities. They want the U.S. Food and Drug Administration to take a greater regulatory role.

But the FDA has said it will do that only on a case-by-case basis — such as the meningitis outbreak that surfaced in Nashville and has since spread to four other states.

Authorities said all of those infected with fungal meningitis received spinal injections of a steroid solution sold by New England Compounding Center, a specialty pharmacy in Framingham, Mass. It and similar facilities combine, mix and/or alter active ingredients to make custom medications, much like early pharmacists.

These modern-day apothecaries now are supplying a greater share of the medicines Americans take, driven by the surge in outpatient procedures, development of bio-engineered drugs and a push for more individualized treatment, experts say.

Estimates vary on how many compounded medications are dispensed. A 2003 Government Accountability Office report cited figures that ranged from 1 to 10 percent of all U.S. prescriptions.

The growth has fueled concerns about the safety and sterility of compounded medications. A 2006 FDA laboratory analysis of compounded medicines found a third were either too potent, not potent enough or had been divided into unequal dosages.

However, the FDA has been selective in its enforcement of compounding pharmacies.

The agency said it targets “firms that manufacture large quantities of unapproved new drugs under the guise of traditional compounding, and whose compounding practices result in significant violations of the new drug, adulteration, or misbranding provisions” of federal drug regulations.

As a result, the responsibility largely falls to state pharmacy boards, whose enforcement capabilities vary.

“They’re not always equipped manpower-wise or financially to survey these pharmacies,” said Allen Vaida, executive vice president of the Institute for Safe Medication Practices, a patient-safety advocacy group based in Horsham, Penn. “The smaller ones … it just seems like they fall into a black hole.”

His group and others have pushed for greater FDA oversight, citing recent outbreaks linked to compounded medications as evidence of the need. Among them:

• A bacterial infection that killed nine people in two Birmingham, Ala.-area hospitals in early 2011. Another 10 people were sickened by the Serratia marcescens bacteria, which was in an intravenous nutritional supplement prepared by a local compounding pharmacy, the New York Times reported.

• Bacteria-tainted Avastin injections that caused eye infections in four patients at the Veterans Administration’s Nashville hospital in March 2011. The hospital had compounded the Avastin for eye injections from packaging intended for intravenous use.

• A fungal endophthalmitis outbreak that struck 33 people in seven states who underwent eye surgery between November 2011 and April 2012, causing vision loss and blindness in some. It was linked to an injectable dye produced by a Florida compounding pharmacy, the FDA said.

The same Florida company, Franck’s Lab, also made an injectable supplement that killed 21 polo horses in April 2009, the Associated Press reported. In that case, Franck’s won a federal court ruling that said the FDA had no jurisdiction over compounding of veterinary medicines given to non-food animals.

But the agency acted against the company again after the endophthalmitis outbreak earlier this year, saying in a July 9 warning letter that “environmental sampling revealed the presence of microorganisms and fungal growth in the clean room where sterile products were prepared.”

As a result, Franck’s stopped selling sterile products — including methylprednisolone, the drug implicated in the Tennessee outbreak — in May, well before the current outbreak, according to FDA records.

Despite those and other high-profile cases, advocates have made little progress in gaining greater federal oversight of compounding pharmacies, Vaida said.

“We feel they should be under FDA control because they are manufacturing,” he said. “Unfortunately, cases like this are what get that effort going.”